The debut feature of writer-director Cary Joji Fukunaga, "Sin Nombre," in Spanish with English subtitles, offers a new variation on a familiar theme. Instead of illegals crossing over the US border from Mexico, Fukunaga shows us Central American migrants trying first to make it through Mexico en route to a better life in America. Sayra (Paulina Gaitan), a Honduran teenager, attempts the life-threatening emigration with her father and uncle by riding the rails. Her story runs into another, more conventionally melodramatic one involving Casper (Edgar Flores), a member of a ruthless Mexican gang who becomes her unwitting protector. Fukunaga has a fine, spacious film sense and a gift for action, but the doomy, heavy-handed plot devices and overwrought, overacted gangland set pieces betray a novice's hand. "Sin Nombre" received raves at this year's Sundance Film Festival. This young director may very well make a marvelous movie one day, but let's not get too carried away right now. Grade: B (Rated R for violence, language, and some sexual content.)